Which cyclization(s) should occur with a decrease in π-electron energy ?
(I) $C H_{2}$ =CH-CH= $C H_{2}$ $\to$ + $H_{2}$
(II) $C H_{2}$ =CH- $C H_{2}^{+}$ $\to$ + $H_{2}$
(III) $C H_{2}$ =CH- $C H_{2}$ $^{∙}$ $\to$ + $H_{2}$
(IV) $C H_{2}$ =CH- $C H_{2}^{-}$ $\to$ + $H_{2}$

Question

Which cyclization(s) should occur with a decrease in π-electron energy ?

(I) $C H_{2}$ =CH-CH= $C H_{2}$ $\to$ structure + $H_{2}$

(II) $C H_{2}$ =CH- $C H_{2}^{+}$ $\to$ structure + $H_{2}$

(III) $C H_{2}$ =CH- $C H_{2}$ $^{∙}$ $\to$ structure + $H_{2}$

(IV) $C H_{2}$ =CH- $C H_{2}^{-}$ $\to$ structure + $H_{2}$

TestHub · Accepted Answer

A decrease in π-electron energy needs the cyclic product to be aromatic. (I) gives cyclobutadiene (4π, anti-aromatic) : energy rises. (II) gives the cyclopropenyl cation (2π, aromatic) : energy falls. (III) the cyclopropenyl radical (3π) is not aromatic. (IV) the cyclopropenyl anion (4π) is anti-aromatic. Only (II).

Chemistry - GOC Question with Solution | TestHub

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